OK, back over this topic.
WYSIWYG has a place. Me personally, I'd never use it, I find the entire idea so imprecise it's unreal (and please don't tell me time's an issue, I make quite literally hundreds of forum posts some days.)
And SMF does have a WYSIWYG editor. Now, if SMF's editor were replaced with another editor, that'd be a different story entirely, because it would have the security issues as mentioned but also the improved functionality of a WYSIWYG editor that wasn't retarded.
Here's an interesting point: we actually discussed a bit back about ditching bbcode and using purely WYSIWYG instead. That's almost more practical than you'd imagine in a lot of ways, it would save a great deal of trouble in terms of processing for a lot of posts, the only real objection I raised wasn't even the implementation of security, or the increased bandwidth use - no, the only objection I raised was in fact concerning usability.
One of the perks of bbcode is that you can introduce quick codes for all kinds of things. Representing a spoiler, or a footnote, in HTML is frustrating if done in a WYSIWYG fashion. However, most users that use WYSIWYG will not use these features.
So, perhaps, there's room for an alternative. For posts made with the WYSIWYG editor, save them with a flag to indicate they came from WYSIWYG. For all other posts, flag them as using bbcode, and render appropriately from the database.
You'd have to be careful about switching between the two, since you would want to do that from time to time, but no more so than is currently being used.